Metal Casting Flashcards
What are the advantages of die casting?
incredible accuracy (high tolerance)
repeatable accuracy
lower failure rate
less waste (cost reduction)
fast process
What are the disadvantages of die casting?
high set up cost
only for lower mp metals (e.g. Al, Zn)
long lead time - have to make complicated moulds
limit on size
Describe the process of die casting.
Preheat dies and coat with heat protectant
pour molten metal into hopper and let it flow into injection mould
inject in at high pressure
let cool
eject with ejector pins and cut off sprue
Describe the process of extrusion casting.
- Die is CNC milled into shape
- Preheat the die to 450-500°C
- Billet is heated and placed into the extrusion chamber
- The heat, pressure and speed is changed for each metal
- The ram applies hydraulic pressure to the billet
- It is pushed through the die and fed onto rollers
- Water is sprayed onto it to cool
- Work hardened by pulling the extrusion
- Cut into smaller pieces
What are the advantages of extrusion casting?
low cost
has a consistent cross-section
flexible
What are the disadvantages of extrusion casting?
product limitations due to it having to be the same size
Describe the process of sand casting
- Make pattern, usually MDF or wood and often split into 2 parts held with dowels. Sloping sides to allow easy removal and filleted edges - much like vacuum forming
- Bottom part of mould (drag) placed on flat surface pattern added. Parting powder might be added to aid pattern removal from mould.
- Fine sand sieved around the pattern to get the smoothest surface finish. To a depth of 30mm.
- Mould than packed tightly with more sand which is “rammed” in the mould to make it compact and solid.
- This is then levelled off.
- Drag turned over and Cope (top part of mould) positioned on top of drag (usually locating pins are included to help them line up)
- Parting powder added to pattern and runner and riser patterns also added to this.
- Facing sand added (sieved sand) and then backing sand rammed around this so compact.
What are the advantages of sand casting?
- Low setup cost - limited tooling required for the casting. Suitable for smaller batch production runs.
- A wide range of metals can be cast, but also able to cast higher melting point metals like steel
- Complex shapes can be produced
- Large components can be produced
What are the disadvantages of sand casting?
- Slower process and sand mould cannot be reused so the labour cost soon mounts up on longer production runs. On very long runs other methods of casting may become cheaper even if their initial tooling is more.
- Surface finish is slightly rough in form due the “gritty nature” of the sand. May require further polishing if a smooth finish is required. Other casting processes (like die casting) will have a higher quality surface finish.
- Moulds can only be used once.
- Pattern needs to be made slightly oversize to allow for the cooling of the metal which shrinks - this can make it tricky to get
need exact sizes for high tolerance parts as the shrink amount can be difficult to work out.
Energy intensive
Describe the process of investment casting
- The first stage of the Investment Casting process is to produce an exact replica (or “pattern”) of the required casting. This is made by injecting wax into a die.
- The wax pattern is assembled onto a wax feeder system. Each assembly or “tree” may have just one large pattern or many small patterns.
- The wax assembly is dipped by a large robot into a high-grade ceramic slurry. This stage is known as the investment stage – the part of the process from which the production method gets its name.
- Whilst the assembly is still wet, larger particles of ceramic are dropped onto its surface. This is done to build up a thicker coating. After drying, this coating process (or “investment” process) is repeated several times, building a shell mould thickness of approximately 10mm.
- After the final shell mould is dried, the wax is melted out in a steam autoclave (hence why the process is sometimes referred to as the “lost wax process”. The moulds are fired at over 1000°C to remove any remaining wax and to strengthen the mould.
- Molten metal is poured into the moulds, whilst they are still hot, enabling the metal to run into thin wall sections.
- After cooling, the ceramic shell mould is broken away and the casting is cut away from the feeder system.
- The castings are carefully fettled, blasted and heat treated. Setting operations may also be required. Material testing, x-ray, dye penetrant testing or machining can then be carried out, if this is required.
What are the advantages of investment casting?
- Can form complex shapes and fine details
- Many material options available for casting.
- High strength parts
- Very good surface finish and accuracy
- Little need for secondary machining
- Capable of casting higher melting point metals compared to die casting because mould material can take far higher temperatures.
- Smaller castings than sand casting.
- Good repeatable accuracy
- Excellent surface finish. Little secondary finishing is required
What are the disadvantages of investment casting?
- More expensive than sand casting for very small runs
- Time-consuming process
- High labour cost
- High tooling cost
- Long lead time possible
- Smaller more intricate components possible compared to sand casting.
- Moulds are not reused so more labour intensive than die casting.
- Wax can be reused in further casting.
- Energy intensive
- Long cooling times.
What is die cutting?
Die-cutting is using metal shapes known as dies in an array of shapes and designs
What are the advantages of die cutting?
mass production process
speed
accuracy
high volumes
only need one die
What are the disadvantages of die cutting?
high set up cost
limited flexibility - has to be the same shape
high wastage
